The invention relates to cathode materials for Li-ion batteries in the quaternary phase diagram Li[Li1/3Mn2/3]O2—LiMn1/2Ni1/2O2—LiNiO2—LiCoO2, and having a high nickel content. Also a method to manufacture these materials is disclosed.
For a long time LiCoO2 (or “LCO”) was the dominating cathode material for rechargeable lithium batteries. LCO has a relatively high capacity (150-160 mAh/g at 3-4.3V) together with a high density (true density about 5.05 g/cm3) and is relatively easy to be produced with good quality. It has a relative high Li diffusion so it is possibly to utilize larger, dense particles (10-20 μm size) with small surface area (0.1-0.5 m2/g). These large, dense, low surface area particles can easily be prepared with a small amount of soluble surface base—as will be defined in more detail below. All in all commercial LCO is a robust and easy to manufacture cathode powder.
LCO is also a “friendly” material during battery production. It is easily processed by battery producer (slurry making, coating, compacting of electrodes, . . . ). It allows to obtain electrodes of low porosity thus more powder can fit into the confined space of a battery ultimatively resulting in a high energy density. For a long time these nice properties ensured the dominance of LiCoO2 for rechargeable lithium batteries for portable applications. LCO however also has serious drawbacks. A main drawback is the relative scarcity of Co resources related to it is the relatively high cost of cobalt metal. Still worse, historically the cobalt prize shows wild fluctuations, and these fluctuations possibly increased the need to find substitutes for LiCoO2.
The main substitute for LCO, which has emerged commercially within the last 5 or so years, is Lithium-nickel-manganese-cobalt-oxide. This material belongs to the quaternary phase diagram Li[Li1/3Mn2/3]O2—LiMn1/2Ni1/2O2—LiNiO2—LiCoO2. Additionally this composition can be modified by doping. It is known for example, that elements like Al, Mg, and sometimes Zr can replace Co, Ni or Mn. Within the complex quaternary phase diagram there is a wide degree of freedom to prepare electrochemically active phases with different composition and quite different performance. Generally, for the production of cathode materials with complex compositions, special precursors such as mixed transition metal hydroxides need to be used. The reason is that high performance Li—M—O2 needs well mixed transition metal cations. To achieve this without “oversintering” the cathode the cathode precursors need to contain the transition metal in a well-mixed form (on atomic level) as provided in mixed transition metal hydroxides, carbonates etc.
When mixed transition metal hydroxides are used as precursors, they are obtained by co-precipitating transition metal sulphates and technical grade bases like NaOH, which is amongst the cheapest industrial route for Li—M—O2 precursor preparation. This base contains CO32− anions in the form of Na2CO3, which are subsequently trapped in the mixed hydroxide—the mixed hydroxide typically containing between 0.1 and 1 wt % of CO32−. Besides the transition metal precursor, the lithium precursor Li2CO3, or a technical grade LiON*H2O, often containing at least 1 wt % of Li2CO3 is used. In the case of high nickel cathodes “LNO”, like LiNi0.8Co0.2O2 when the lithium hydroxide and transition metal precursors are reacted at high temperature, typically above 700° C., the Li2CO3 impurity remains in the resulting lithium transition metal oxide powder, especially on its surface. When higher purity materials are used, less Li2CO3 impurity is found, but there is always some LiOH impurity that reacts with CO2 in the air to form Li2CO3. The use of high purity materials is proposed in JP2003-142093, however such expensive precursors are not preferred.
It is convenient to simplify the quaternary phase diagram when discussing phase relations. In the following we will use the ternary phase diagram to discuss properties of layered LiMO2 cathodes: X*LiCoO2+Y*LiNi1/2Mn1/2O2+Z*LiNiO2, with X+Y+Z=1.
When X increases the consequences are:                raw materials cost increase (high cost of Co),        production usually becomes more easy, and        rate performance improves.        
When Z increases:                production becomes more difficult,        reversible capacity increases,        air stability decreases,        content of soluble base increases, and        safety deteriorates.        
The cathodes in this diagram can have the general formula Lia((Niz(Ni1/2Mn1/2)yCox)1−kAk)2−aO2, wherein x+y+z=1, and A is a dopant, with 0≦k≦0.1, and 0.95≦a≦1.05. The values of x,y,z correspond to the above described values of X, Y and Z respectively.
Within the ternary phase diagrams commercially interesting members include “NCA” (e.g. LiNi0.8Co0.15Al0.05O2) or “111” (e.g. Li1.05M0.95O2 with M=Ni1/3Mn1/3Co1/3) or “532” (e.g. LiMO2 with M=Ni0.5Mn0.3Co0.2). These different members have very different properties. NCA for example, has a very high capacity, by far exceeding LiCoO2. But it is very difficult to prepare since CO3 free precursors—like purified LiOH—are needed, and a CO2 free reaction atmosphere—like oxygen—and the final cathode material is sensitive to moisture exposure. It is unstable during long air exposure and it typically has a very high content of soluble base. On the other hand, “111” is very easy to prepare, for example from a mixed precursor and Li2CO3. The resulting cathode is very robust and the content of soluble base is small. However, the reversible volumetric capacity is lower than LiCoO2 and sometimes even the rate performance is insufficient.
As discussed in U.S. Pat. No. 7,648,693, the content of soluble base obtained for a given cathode powder can be determined in a reproducible manner by pH titration, which depends on parameters such as the total soaking time of powder in water. Bases are originating mainly from two sources: first, impurities such as Li2CO3 and LiOH present in the Li—M-O2; second, bases originating from ion exchange at the powder surface: LiMO2+δH+←→Li1−δHδMO2+δLi+.
Recently cathode materials with compositions like “532” (X=Z=0.2) became very serious challengers to LCO. Whereas “111” (X=⅓, Y=⅔, Z=0) and “NCA” (Z=0.8) did not successfully substitute LCO in commercial applications the situation is different for “532”. “111” was not successful (Z too low) due to low energy density (a battery with 111 has lower capacity than the same design with LCO), NCA was not successful by the opposite reason (Z much too high) because of high content of soluble base, high sensibility and relatively high production cost. However, “532” has higher capacity than “111” and is much more robust than “NCA”. The production is more difficult than “111” but much cheaper than “NCA”. So “532” allows to substitute LCO without loosing energy density, and a cheap production process is manageable.
An essential requirement to successfully replace LiCoO2 is a cheap and simple production process. Preferably—as lithium precursors—lithium carbonate is used and the firing is done in normal air. At the same time, there is a strong trend to increase “Z” thus pushing the energy density further up. The current art however teaches that there is a limit to increase Z using such a process: in US2006/0233696 for example it is said that, for Z>0.35, the doped LiNiO2 cannot be prepared in air on a large scale and Li2CO3 cannot be used as a precursor. That is because this document believes that a good Ni-based lithium transition metal oxide can only be obtained when it is substantially free of soluble bases. It is an aim of the current invention to go beyond this belief and develop methods to achieve good quality products with higher Z values whilst using low cost technical grade precursor materials. If a high capacity, good performance and low price cathode material is available it will very much boost the substitution of LCO in all applications. But: “High Z” materials prepared by a cheap process often have a high content of soluble base. This problem is also addressed in the present invention.
To summarize, a production process is needed which allows to produce “high Z” materials having a good quality with a low (but not too low—as will be discussed below) content of soluble base, and at low cost. Particularly the base content of soluble base needs to be optimized without jeopardizing performance and production cost.